1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotography such as a full-color copying machine, a printer, a facsimile apparatus and a composite apparatus thereof, and more particularly to an image forming apparatus having a rotary developing device, in which the total size of the image forming apparatus is reduced, and a space for disposing a drive source for rotationally driving the rotary developing device is kept.
2. Description of the Related Art
Conventionally, the image forming apparatuses of electrophotography such as a full-color copying machine, a printer, a facsimile apparatus and a composite apparatus thereof are based on a so-called tandem system is well known in which a plurality of image forming portions corresponding to each color of yellow, magenta, cyan and black are arranged in parallel, a toner image of each color of yellow, magenta, cyan and black formed by each image forming portion is directly transferred onto the recording sheet, or secondarily transferred via an intermediate transfer belt onto the recording sheet to form a full-color image.
Also, some image forming apparatuses such as the full-color copying machine, printer and so on are based on a so-called four cycle system in which a plurality of developing units corresponding to each color of yellow, magenta, cyan and black are arranged adjacent to a single photoconductive drum along the circumferential direction of a developing device main body, the developing device main body is rotationally driven to move a developing unit of each color to a developing position opposed to the photoconductive drum, a toner image of each color of yellow, magenta, cyan and black is formed on the photoconductive drum by rotating the photoconductive drum a predetermined number of times, and the toner image of each color of yellow, magenta, cyan and black formed on the photoconductive drum is directly transferred onto the recording sheet, or secondarily transferred via the intermediate transfer belt onto the recording sheet.
Among the image forming apparatuses of the tandem system and the four cycle system, the image forming apparatuses of the four cycle system have a feature of having one photoconductive drum and the small size.
By the way, some image forming apparatuses of four cycle system are well known in which an image forming member is laid out in consideration of the smaller size and operability of the apparatus, as disclosed in JP-A-2001-175077 and JP-A-2002-341706, for example.
The image forming apparatus as disclosed in JP-A-2001-175077 and JP-A-2002-341706 has a rotary developing device 101 in which a plurality of developing units 102 corresponding to each color of yellow, magenta, cyan and black are arranged along the circumferential direction sideways of or obliquely under a photoconductive drum 100 of smaller diameter, to develop an electrostatic latent image of each color of yellow, magenta, cyan and black formed on the photoconductive drum 100, as shown in FIGS. 14 and 15. Though the rotary developing device 101 itself is made as small as possible, it is required that the developing units 102 corresponding to each color of yellow, magenta, cyan and black are arranged. Therefore, it is difficult to make the apparatus smaller than a predetermined size, and the developing units occupy a fixed percentage of space in the image forming apparatus main body 103.
Consequently, to reduce the total size of the image forming apparatus, it is required to make the image forming member other than the rotary developing device 101 as small as possible, and contrive the lay-out of the image forming member. For example, the photoconductive drum 100 can be formed in relatively small size, namely, small diameter. Also, when the full-color image is formed, it is necessary to provide an intermediate transfer body 104 such as an intermediate transfer belt onto which the toner image of each color of yellow, magenta, cyan and black formed sequentially on the photoconductive drum 100 is transferred multiply. It is required that the intermediate transfer belt 104 has the minimum circumferential length corresponding to the size of recording sheet for recording the full-color image, whereby the reduction of size is limited. Since the intermediate transfer belt 104 is a belt member having flexibility, the cross-sectional shape for stretching intermediate transfer belt can be set arbitrarily. Also, in the image forming apparatus, an image exposure device 104 for exposing the image on the photoconductive drum has also a predetermined size.
In this case, in the image forming apparatus as disclosed in JP-A-2001-175077 and JP-A-2002-341706, the intermediate transfer belt 104 is disposed on top of the photoconductive drum 100, and the rotary developing device 101 and the image exposure device 105 are disposed sideways of or obliquely under the photoconductive drum 100, as shown in FIGS. 14 and 15.
Also, in the image forming apparatus as offered in the above document, the distance from the primary transfer position of the intermediate transfer belt 104 to the secondary transfer position is relatively long, because the image exposure device 105 is disposed.
Moreover, in the image forming apparatus as offered in the above document, the rotary developing device 111 is disposed closely to one side of the image forming apparatus main body 103 to replace each developing unit 102 of the rotary developing device 101.
However, there were the following problems with the prior art. That is, in the image forming apparatus as disclosed in JP-A-2001-175077 and JP-A-2002-341706, the rotary developing device 101 is disposed closely to one side of the image forming apparatus main body 103 to replace each developing unit 102 of the rotary developing device 101, as shown in FIGS. 14 and 15. Also, in the image forming apparatus, because the image exposure device 105 is disposed on the opposite side of the rotary developing device 101, a space for disposing the image exposure device 105 must be kept to leave a large obliquely under the photoconductive drum 100, and because the conveyance passage for conveying the recording sheet is disposed, the transfer belt 104 to the secondary transfer position is longer, resulting in a problem that the reduction of size is limited.
Also, in the image forming apparatus as disclosed in JP-A-2001-175077 and JP-A-2002-341706, the drive motor 106 for rotationally driving the rotary developing device 101 is disposed on the back side of the image forming apparatus main body 103, but because the rotary developing device 101 is greatly larger in weight and diameter than the photoconductive drum 100, it is required to employ the drive motor 106 having the largest moment of inertia in the image forming member and a large size. Consequently, in the image forming apparatus, the large drive motor 106 protrudes on the back side of the image forming apparatus main body 103, resulting in a problem that the image forming apparatus main body 103 is not made thin.
In recent years, there is a strong demand for the image forming apparatus of small size and thin type with a smaller depth (in the direction of rotational axis of the rotary developing device), because the image forming apparatus is installed on the desk when in use, in which there is a problem that the image forming apparatus of thin type can not be realized with the techniques as disclosed in JP-A-2001-175077 and JP-A-2002-341706.